<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 600px;
            margin: 100px auto;
        }
        .message{
            display: inline-block;
            font-size: 12px;
            color: #999;
            background: url(../images/mess.png) no-repeat left center;
            padding-left: 20px;
        }
        .wrong {
            color: red;
            background-image: url(../images/wrong.png);
        }
        .right {
            color: green;
            background-image: url(../images/right.png);
        }
    </style>
</head>
<body>
    <div class="regist">
        <input type="password" class="ipt">
        <p class="message">请输入6-16位密码</p>
    </div>
    <script>
        //1.获取元素
        var input=document.querySelector('.ipt');
        var message= document.querySelector('.message');
        //2.注册事件  失去焦点
        input.onblur = function(){
            // 根据表单里面值的长度  ipt.value.length
            if(this.value.length<6||this.value.length >16){
                // console.log('错误');
                message.className = 'message wrong';
                message.innerHTML = '您输入的位数不对要求6~16位';
            }else {
                message.className = 'message right';
        }
    }
    </script>
</body>
</html>